The last four caldera-forming ignimbrites in the central San Juan caldera cluster, Colorado, erupted 1400 km3 in ≤80 kyr and alternated between zoned crystal-poor rhyolite to crystal-rich dacite and unzoned, crystal-rich dacite. The zoned 150 km3 Rat Creek Tuff (26·91 Ma), unzoned 250 km3 Cebolla Creek Tuff, and zoned 500 km3 Nelson Mountain Tuff (26·90 Ma) formed the nested San Luis caldera complex with slightly offset calderas, and the unzoned 500 km3 Snowshoe Mountain Tuff (26·87 Ma) formed the Creede caldera to the south. The Rat Creek Tuff, Nelson Mountain Tuff, and Snowshoe Mountain Tuff have similar mineral assemblages of plagioclase, sanidine, quartz, biotite, hornblende, clinopyroxene, Fe–Ti oxides, and accessory zircon, titanite, and apatite. The Cebolla Creek Tuff differs from the other three ignimbrites with more abundant hornblende and a lack of quartz and sanidine. Trace element compositions of interstitial glass are unique to each ignimbrite, correlating with mineral assemblages and inferred crystallization depths. Glass, feldspar, hornblende, and clinopyroxene thermobarometry calculations provide evidence for vertically extensive crustal magma reservoirs with a common magmatic zone at ∼435–470 MPa (∼16–17 km) showing a transition into shallow pre-eruptive reservoirs between ∼110 and 340 MPa (∼4–13 km), similar to the estimated magma reservoir architecture of the Altiplano Puna Volcanic Complex. The upper portions of the eruptible parts of the magma reservoirs of the Rat Creek Tuff (215 ± 50 MPa, ∼810–820 °C), Cebolla Creek Tuff (340 ± 20 MPa, ∼860–880 °C), Nelson Mountain Tuff (215 ± 20 MPa, ∼745–800 °C), and Snowshoe Mountain Tuff (110 ± 40 MPa, 825 ± 10 °C) occupied shallow levels in the crust similar to other magma reservoirs of the central San Juan caldera cluster. Trace element modelling correlates with a deep crystallization signature in the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff, typified by a flat trend in Ba versus Sr whole-rock and glass chemistry. The zoned Rat Creek Tuff and Nelson Mountain Tuff are typified by a steep trend in Ba versus Sr chemistry interpreted as a shallower crystallization signature. Similarly, the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff have flatter slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant deep plagioclase crystallization and a difficulty in physically mixing with Fe-rich mafic recharge magma owing to higher viscosity. The zoned Rat Creek Tuff and Nelson Mountain Tuff have higher slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant shallow plagioclase crystallization and more feasible mixing with Fe-rich mafic recharge magma owing to lower viscosity. The eruption of the Rat Creek Tuff was probably triggered by mafic injection, but the other three ignimbrites lack mingling textures in pumice, suggesting that other mechanisms were important in causing such large eruptions. After a prolonged period of mantle-derived magma injection and crustal heating (∼25 000 km3 Conejos Formation erupted during ∼35–29 Ma), the San Juan magmatic body became a robust and versatile producer of diverse eruptible magmas in short time periods during its Oligocene ignimbrite flare-up.

中文翻译：

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分区和未分区 Ignimbrites 的岩浆演化：科罗拉多州圣胡安山脉的四个快速序列、形成火山口的喷发形成复杂地壳结构的证据

科罗拉多州圣胡安火山口群中部的最后四个形成火山口的火成岩在 ≤80 kyr 内喷发了 1400 km3，并在带状晶体贫乏的流纹岩和富含晶体的英安岩和未分区、富含晶体的英安岩之间交替。划定的 150 km3 Rat Creek 凝灰岩（26·91 Ma）、未划定的 250 km3 Cebolla Creek 凝灰岩和划定的 500 km3 Nelson Mountain Tuff（26·90 Ma）形成了嵌套的 San Luis 破火山口复合体，破火山口略有偏移，未划定的 500 km3 雪鞋山凝灰岩（26·87 Ma）形成了南部的克里德火山口。Rat Creek 凝灰岩、尼尔森山凝灰岩和雪鞋山凝灰岩具有相似的斜长石、山尼丁、石英、黑云母、角闪石、单斜辉石、Fe-Ti 氧化物和副锆石、钛铁矿和磷灰石的矿物组合。Cebolla Creek 凝灰岩与其他三种火成岩不同，具有更丰富的角闪石，缺乏石英和山宁石。间隙玻璃的微量元素组成对于每种点火岩都是独一无二的，与矿物组合和推断的结晶深度相关。玻璃、长石、角闪石和单斜辉石热气压计计算为垂直分布广泛的地壳岩浆储层提供了证据，其共同岩浆带在约 435-470 兆帕（约 16-17 公里），显示向约 110 至 340 之间的浅层喷发前储层过渡MPa（~4-13 km），类似于Altiplano Puna火山复合体的估计岩浆储层结构。Rat Creek Tuff (215 ± 50 MPa, ∼810–820 °C), Cebolla Creek Tuff (340 ± 20 MPa, ∼860–880 °C) 岩浆储层可喷发部分的上部，Nelson Mountain Tuff (215 ± 20 MPa, ∼745–800 °C) 和 Snowshoe Mountain Tuff (110 ± 40 MPa, 825 ± 10 °C) 占据地壳中的浅层，类似于圣胡安中部火山口的其他岩浆储层簇。微量元素建模与未分区的 Cebolla Creek 凝灰岩和雪鞋山凝灰岩中的深度结晶特征相关，典型的特征是 Ba 与 Sr 全岩和玻璃化学的平坦趋势。划区的 Rat Creek 凝灰岩和 Nelson Mountain 凝灰岩的典型特征是 Ba 与 Sr 化学的陡峭趋势被解释为较浅的结晶特征。相似地，未分区的 Cebolla Creek 凝灰岩和雪鞋山凝灰岩在 Fe2O 中具有更平坦的斜坡，而斜长石化学空间被解释为更丰富的深斜长石结晶，并且由于粘度较高而难以与富含铁的镁铁质补给岩浆物理混合。分区的 Rat Creek 凝灰岩和 Nelson Mountain 凝灰岩在 Fe2O 中的斜率高于斜长石化学空间，这被解释为更丰富的浅斜长石结晶，并且由于粘度较低，更容易与富含铁的镁铁质补给岩浆混合。Rat Creek 凝灰岩的喷发可能是由镁铁质注入引发的，但其他三种火成岩在浮石中缺乏混合质地，这表明其他机制在导致如此大规模的喷发中很重要。